262 bulletin: museum of comparative zoology. 



shift the lamp until the wave-lengths read 420-480 fxfx. This was 

 then repeated for the light proceeding from generator B. From what 

 has been said, the radiant energy in the blue light from generator B 

 must now be approximately equal to that in the blue light from genera- 

 tor A. The boxes were then so revolved on the pivots (P and P', Fig. 

 1), that the light rays from the two were parallel and opposite. A 

 photometer placed midway between the two boxes in the path of the 

 lights then showed that the intensity of the two lights was equal. 

 The lights were now in such a condition that the animals could be 

 introduced for tests. Under the separate sections the special methods 

 used and the procedure of the tests themselves will be more fully given. 



It may seem to have been unnecessary to have gone to all of this 

 trouble, when the wave-lengths of the lights from generator A could 

 have been read, and the animals tested in this. But the use of two 

 lights of the same wave-lengths from opposite sources, lessened the 

 chance of experimental error. Under the description of the reactions 

 to single monochromatic lights, it will be shown how the method of 

 rotation and the use of the two lights from opposite sources were 

 combined to offset the influence of compensating movements upon 

 the reactions. The procedure for the blue, just described, was 

 repeated for each of the other three sets of colored lights. 



When the reactions to balanced pairs of monochromatic lights 

 were tested, the procedure, while in the main the same as that just 

 described, differed somewhat from it. Here there were two lights 

 from different sources, midway between which the animal was placed, 

 so that the lights impinged at right angles upon opposite sides of its 

 body. Further, the lights were of different wave-lengths, except in a 

 few experiments that were carried out to check the equality of the 

 lights from the two light-generators. For the sake of clearness in 

 explaining the procedure, a concrete case again will be taken. Sup- 

 pose, for example, that it was desired to test the reactions of a lot of 

 toads in blue and in red light. The single-glower lamps were placed in 

 both boxes, lighted, and the appropriate diaphragms placed in position. 

 The light proceeding from generator A was then scrutinized in a 

 spectroscope, and the lamp adjusted until it read 630-655 /x/x, which 

 were the wave-lengths of the red light used. The same process was 

 repeated for generator B. The energy contained in the two red lights 

 was then assumed to be the same. The boxes were then so revolved 

 on the pivots (P and P', Fig. 1), that the light rays from the two 

 were parallel and exactly opposite. A photometric reading showed 

 that the intensity of the two lights was approximately equal. The 



